In digital color publishing applications it is often desirable to distribute the rendering of a job on multiple devices which may or may not be physically co-located. In this patent, the term “devices” generally refers both hardcopy devices (i.e. printers) and softcopy display devices. For example, in cluster printing a color job might be split among multiple co-located printers in order to meet deadlines, reduce cost, or optimize overall print shop capacity.
Distributed printing from a centralized repository close to the final site of delivery is another scenario where rendering is split among multiple printers; which are not physically co-located. It will often be crucial that color reproduction amongst separate devices be highly consistent as color characteristics vary widely across devices and device controllers. Proper color management is thus needed to ensure color consistency.
One approach is to associate color correction (e.g., ICC) profiles with each output device. The profiles are derived independently for each device and loaded statically into the job management system. The colors of the input job are mapped to a device-independent color space (e.g., CIELAB) and color-corrected to the output device's profile prior to rendering. Such an approach can be found in U.S. Pat. Nos. 6,043,909 and 6,157,735 wherein a system for controlling and distributing color in a networked environment is disclosed. Both teach the concept of a “Virtual Proof”, an abstract data structure that contains and manages the color profiles for each device in the system as well as the associated color-correction transformations to be applied to the input job. Although the use of device-independent color specification and profiles for color rendition on an output device is an improvement in the arts for device specific representation, this does not guarantee consistent color reproduction in certain applications involving multiple output devices.
Another problem arises from the fact that different output devices have different color gamuts. The gamut of an output device is defined as the region of colors in a device independent color space that can be reproduced on that device. In addition, the effective color gamut of a printer is often dependent on the various choices of image path elements such as ink-limit, gray component replacement (GCR), and halftones in instances where printers with different sets of image path elements represent different output devices. Standard color management approaches can only guarantee consistent color reproduction for colors in the job that are already within a color gamut common to all the output devices. The common gamut is the intersection of the individual device gamuts computed in a device independent color space. It is common for jobs to contain colors outside this common gamut. For example, consider a business graphic containing the primary colors of a display to be reproduced on multiple printers. Typically these colors are outside the gamut of all the printers and the application of independent color correction transforms does not guarantee consistent output among the devices. Differences can also be seen in saturated colors in pictorial images.
One potential solution to the problem of color consistency across multiple devices is to define a universal consistent color mode for all devices that ensures consistency across the different devices. For example, a universal consistent color mode may be achieved by restricting the colors for all output devices to the common gamut of the universe of devices employed. In order to be more useful, temporal variations among devices and differences across devices should be comprehended in computing the common gamut. Color critical jobs may then be rendered using the consistent mode to ensure that some inter-device differences do not unduly affect the color rendering of the job. This approach however has several limitations. One is that the restriction to the common gamut over time and across devices often exacts an unnecessary penalty in image quality. Even for a single device family, a significant region of the dynamic range may need to be sacrificed in order to achieve consistency over the fleet and over time. In addition, this does not scale well as new devices are introduced or older devices are removed. The introduction of a new device or removal of an existing device often requires an upgrade of the “consistent-mode” corrections at all existing devices. Lastly, upon re-calibration and re-characterization of a device, each existing device should be updated.